Report on SNEAP’08 and notes about the ECool’s Pelletron

1. Report on SNEAP’08and notes about the ECool’s Pelletron Recycler Departmental Meeting October 22, 2008 A. Shemyakin

2. SNEAP • Symposium of Northeastern Accelerator Personnel • “a community of personnel involved with electrostatic particle accelerators and their use” • Founded in 1968 by three Northeastern universities • The name is kept for historical reasons • Annual meetings • Exchange of mostly very practical information • everything relevant to the operation of electrostatic accelerators • electrostatic accelerators, ion sources, telemetry and control systems, maintenance, safety issues etc. • “use RAG IT-II wipes to clean the tank, it improves reliability” • Accelerator physicists, engineers, technicians • SNEAP’2008 • 32 participants • At maximum, in 80’s, SNEAP had more than 100 participants • People from USA, Australia, Italy, France, Canada, New Zealand, Finland • Reports and a lot of discussions

3. Trends (G. Norton, NEC) • Growth of voltage/energy culminated with installation of ORNL’s 25 MV Pelletron tandem in 1976 • 25.5 MV with beam (1988) • The only machines that reliably runs with beam above 20 MV • Nuclear physics • Breaking the Coulomb barrier for heavier nuclei • No new machines above 10 MV for ~15 years • Dozens are used for nuclear physics for many years • Knowledge how to manufacture is getting lost • NEC keeps changing the Pelletron design/fabrication • Now all acceleration tubes are high-pressure rinsed with distilled water • After proposal and tests at JAERI (Japan) • Installing the better conductivity chain sheaves to eliminate conductive bands • Better software • The last Pelletron in Pakistan was commissioned remotely • Improved terminal electronics

4. Trends (cont.) • Most of new Pelletrons are < 3 MV • Horizontal tandems • Atomic Mass Spectrometry • 14C/12C, 10Be/9Be, 26Al/27Al,… • Background 10-16, 1 mg samples, 2000 samples in 5 days (2% precision), ~70 labs • Fast growing field is Biomedical applications • Material analysis • Rutherford backscattering, Proton-induced X-ray emission,… • Material modifications and tests • The main stress is on the design of equipment outside the accelerator • Accelerators are highly reliable and are key-ready • True unattended operation • Beam handling and analysis are more and more sophisticated • Some of older accelerators run for many years without opening • UEC Inc.’s 1.7 MeV: twice in 12 years • Alabama A&M: once per 5 years • Decreases the need for simple replicas of older accelerators • Machines work for decades, are transferred between labs, are modified

5. Specifics • Many of advances come from high voltage (> 10 MV) machines • Larger groups, more frequent openings, more efforts are required to provide a reliable operation • ANU (14 MV): 109 openings in 35 years; lately once in 10 months • Almost all machines are with positive terminal voltage • Needles are at the ground as well as all HV regulation circuitry • Slow travel time for electrons to the terminal => limits the frequency response • Often only stripping foil or gas are in the terminal • Negative ion source is at the ground potential • Usually determines reliability • Electronics protection depends on the amount of stored energy • Minimum at < 2 MV • Steel boxes at > 3 MV • Double shielding at >10 MV • ECool’s Pelletron ran between 2007 and 2008 shutdowns ~92% of time • ~8000 hrs/year • More than other machines (1000 – 5000)

6. Practical experience (relevant for us) • GVM • Motor or GVM failures are a common cause for tank openings • Nobody has two GVM • Bearing replacement is considered a routine procedure • Some have 4” GVMs, but with 4 blades (from HVE) • GVM problems may be related to bad bearings, dust, grease from bearings… • Energy stability is typically by at least several times worse than ours • To catch coming bearing failures, one can carefully listen to the noise during openings • Typically the chain and the shaft can be rotated with people in the tank • In large machines, there are switches to turn them on/off from inside the tank • Chain quality can controlled by a dedicated capacitive pickup sensitive to the charge of individual pellets • Make one of the pellets not-chargeable, use it as a marker, and look at evenness of charges • Can replace individual pellets • Nobody reported about replacements of the chain or shaft motors

7. Gas quality • A large absolute amount of oxygen may result in a fire • One the main reasons that prevents of using just a compressed air • Reliability depends critically on the gas quality • Life time of bearings and chains • Difficult to analyze SF6 byproducts • If a sample is taken with SS cylinder (as we are doing), the byproducts quickly react with walls • ANU (Australia’s 14 MV Pelletron) developed a dedicated device • Notre Dame: • Replacement of the old N2/CO2/SF6 gas by a new N2 + 10%SF6 increased dramatically the chain life time • Typical pressure is 100 -120 psia • One of checks is the weight of a bottle with liquefied SF6 • Air stays gaseous

8. ANU: chain life time CHAIN LIFETIME ENVIROMENTAL PERIODS • 1, 1973-1983. CORONA PTS, STANDARD PURIFIER, HIGH BREAKDOWN PRODUCT CONTENT. TYP HOURS 4000 • 2, 1983-1988. CORONA PTS, ENLARGED PURIFIER, LOW BREAKDOWN PRODUCT CONTENT. TYP HOURS 25000 • 3, 1988-TO PRESENT. RESISTORS INSTALLED, STANDARD PURIFIER, LOW BREAKDOWN PRODUCT CONTENT. AT +77000 HOURS • PELLET GAP CHAIN 3 BETWEEN 6.5 AND 7.5 mm; GAP ON A NEW CHAIN IS 5.4 mm • THIS PIN IS 0.010” SMALLER ON RADIUS. ECooler’s chains: ~15 000 hrs

9. History of HV – related failures in Fermilab’s cooler • ~ 5 years with no failures • 5 failures in the last year • There may a reason for the worsened reliability

10. SF6 analysis in the ECool’s Pelletron (%) • No water is found • In agreement with the low dew point (< -80 C) reported by a sensor on SF6 skid • No SF6 byproducts • Could be related to the test procedure • A large amount of oxygen and nitrogen is reported • A 4th sample has been taken in Oct 08 and is being analyzed Steady leak ~2 psi/year • Events • SF6 was purified while moving into MI-31 in 2004 • May 2006 – new SF6 blower • Leak rate ~2 psi/year since then • 17-Feb-08 – beginning of high leak rate (~2 psi/month) 25-Oct-06 22-Oct-08 Two year history of SF6 pressure and temperature in the tank

11. Discussion • In 2006 -2007, SF6 loss did not correlate with adding air into the system • The leak may be in the new blower assembly which always stays at a positive SF6 pressure • Significant degradation of SF6 quality occurred during the year when a large leak through tank flanges occurred • The air might go in during the SF6 vacuum pumping stage • The leak has been repaired • The equilibrium pressure is 5 mTorr • At the time of SF6 leak repair, the vacuum gauge was replaced • Before the repair, the (previous) gauge read 300 mTorr • If the report is correct, the problems with reliability might be caused by oxygen • We should think hard about possible sources of the contamination • It might have been resolved by the leak repair • We may think about changing the procedure to send some of SF6 to atmosphere when the tank pressure is deeply negative • A gas purification needs to be scheduled for the April 2009 shutdown • Lengthy process (a week) • May need to add 1/3 of the total SF5 mass • Shall we consider N2/SF6 mixture?